In recent years, there has been a rapid progress in the research and development of a high-density optical disk system capable of recording and/or reproduction (hereinafter referred to as “recording/reproduction) of information, using a blue-violet semiconductor laser having a wavelength of about 405 nm. For example, an optical disk for conducting information recording/reproduction based on NA of 0.85 and light source wavelength of 405 nm—a so-called Blu-ray Disc (hereinafter referred to as “BD”)—is capable of recording 23 through 27 GB information per layer, as compared to the optical disk having a diameter of 12 cm, the same diameter as that of the DVD (NA: 0.6; light source wavelength: 650 nm, memory size: 4.7 GB). Further, an optical disk to conduct information recording/reproduction based on NA of 0.65 and light source wavelength of 405 nm—so-called HD DVD (hereinafter referred to as “HD”)—is capable of recording 15 through 20 GB information per layer as compared to the optical disk having a diameter of 12 cm. When the BD is used, there is an increase in the coma aberration caused by the skew of the optical disk. In the present Specification, such an optical disk will be referred to as a “high-density optical disk”.
The value of the optical disk player/recorder as a commercial product such as optical disk player or recorder (hereinafter referred to as “optical disk player/recorder) cannot be said to be sufficient in some cases if information recording/reproduction can be conducted only when a high-density optical disk is used. At present, since the DVD and CD (compact disks) for recording a great variety of information are put on the market, the commercial value of the optical disk player/recorder for high density optical disk is enhanced if adequate information recording/reproduction capacity is ensured for the DVD and CD owned by the user, for example. Thus, the optical pickup apparatus mounted on the optical disk player/recorder for high-density optical disk is desired to ensure adequate information recording/reproduction for any of the high-density optical disk, DVD and CD.
A method has been proposed to permit adequate information recording and/or reproduction while maintaining compatibility with any of the high-density optical disk, DVD and CD. According to this method, selective switching is carried out between the optical system for high-density optical disk and the optical system for DVD and CD in response to the recording density of the optical disk for information recording and/or reproduction. However, this method requires a plurality of optical systems, and is disadvantageous from the viewpoint of downsizing and cost cutting.
To simplify the structure of the optical pickup apparatus and to reduce the cost, it is preferred in the optical pickup apparatus characterized by compatibility that standardization should be achieved between the optical system for high-density optical disk and the optical system for the DVD and CD wherever possible, thereby minimizing the number of the optical parts constituting the optical pickup apparatus. Further, standardization of the objective optical system laid opposite to the optical disk is the shortest way to simplify the structure of the optical pickup apparatus and to reduce the cost.
The Patent Document 1 discloses an optical pickup apparatus wherein a diffraction structure is employed to maintain compatibility between a DVD and CD in information recording and/or reproduction.
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 2005-141800
However, the optical pickup apparatus of the Patent Document 1 fails to provide adequate compatibility between the high-density optical disk and DVD in information recording and/or reproduction. The thicknesses of protective layer and the wavelength in the optical disks intended for compatible use are different between the high-density optical disk and DVD, and between the DVD and CD. This makes it difficult to ensure compatibility in use and correction of chromatic aberration cannot be ensured.
The following specifically describes the compatibility in use: The objective lens and collimator in the Patent Document 1 is provided with a diffraction structure for emitting the diffracted light highly efficient for the light of the DVD and CD waveforms. When the light of a high-density optical disk has entered, highly efficient diffracted light is emitted. However, since the diffracted light enters the objective lens at the angle of divergence almost the same as that of the CD wavelength light, the objective lens excessively corrects the spherical aberration generated by the thickness of the protective layer of the high-density optical disk. This arrangement fails to achieve the compatibility in use. Further, the following problem is found in the correction of chromatic aberration. In the refractive objective lens, the light of the high-density optical disk wavelength has a chromatic aberration 2.5 through 3.5 times greater than that of the CD wavelength. Thus, in the optical system optimized to the light of CD wavelength, chromatic aberration remains uncorrected.